- shorter.
- congruent with `nixos-rebuild .` choosing what to build based on `hostname`.
- more widely used within other nix repos i've seen.
- more accurate in the case that i migrate a host to a different
machine (which i plan to do with servo).
the root part is ext4 instead of btrfs. nixos-generators doesn't support
btrfs. the underlying machinery does though, so we can remove the
middleman in a future patch to achieve that.
